In parallel with an increasing tendency toward the miniaturization of integrated circuits in the electronic industry, there has been an increasing trend aimed at the production of circuit boards which permit high density mounting to take full advantage of the miniaturization of the devices. Also, there has been a tendency toward forming both a power supply circuit and signal circuits on a single board.
In the past, printed wiring boards have been used in which a plurality of double-sided copper clad boards each were formed with a printed wiring pattern on laminated foil copper by the subtractive process. These boards were laminated, and the necessary conductive passages from layer to layer provided by plated through holes. However, the boards' structures and the dielectric resins were typically made for low density applications.
A further difficulty with known methods of production for multilayer boards is that with the trend toward higher and higher densities although the board gets thicker, the size of the plated through holes gets smaller. The present technology requires hole diameters of under 0.005 inch. Drilling these holes is not possible with the current technology, and maintaining the tight tolerances on the size of the holes has reached the limits of machine capabilities. At present, the limit of conventional hole sizes is 0.015 inch, with a hole pad of 0.03 inch diameter. Thus, the problems to be addressed in fabrication of multilayer double-sided boards include keeping the finished board thin, maintaining small hole size for through holes, while avoiding the stresses created by the heat of pressure of lamination.